fix: handle utf-8 in windows sandbox logs (#8647)

Currently `apply_patch` will fail on Windows if the file contents happen
to have a multi-byte character at the point where the `preview` function
truncates.

I've used the existing `take_bytes_at_char_boundary` helper and added a
regression test (that fails without the fix).

This is related to #4013 but doesn't fix it.

codex-rs/Cargo.lock

@@ -2006,6 +2006,7 @@ dependencies = [
  "chrono",
  "codex-protocol",
  "codex-utils-absolute-path",
+ "codex-utils-string",
  "dirs-next",
  "dunce",
  "pretty_assertions",